When motors are connected to an electrical supply network, in particular with a direct start, but also with a star-delta start, significant starting currents occur. In the case of a direct start, these currents can be six to eight times the rated motor current and, in the case of star-delta start, 3.5-4.5-times the rated motor current. Twice the motor currents still occur with soft starters or frequency converters. However, even when other electrical consumers such as, for example, capacitors in compensation systems or lighting strips in factory buildings are connected, there is greatly increased power consumption for a brief period.
In addition to the size of these currents, the duration of such overloads is also of significance. With capacitive and resistive loads, these are typically from less than 10 to a maximum of 100 ms. In the case of motors, this time constant is much longer. In an ideal case, the start-up time can be less than 200 ms but it can also be up to a few 10 s (worst-case start-up time).
Both these core variables have a considerable influence on the load on the electrical supply network and on the network quality of the supply network (voltage dips). In particular in the case of the parallel connection of a plurality of (large) consumers, the feed-in power of the supply network can be too low. The voltage dips that occur then can result in faults in components or overloads.
Power supplies are therefore designed with certain simultaneity factors which are typically less than 80% of all consumers at rated load.